A networked storage system may include a number of storage appliances. A storage appliance may provide services related to the organization of data on mass storage devices, such as disks (e.g., magnetic disks, optical disks). Certain types of storage appliances are commonly referred to as filers or file servers. An example of such a storage appliance is any of the products made by Network Appliance, Inc. in Sunnyvale, Calif. A storage appliance may be implemented with a special-purpose computer or a general-purpose computer programmed in a particular way. Depending on the application, various networked storage systems may include different numbers of storage appliances.
To mitigate the impact of loss of data in the event of a disaster, data is routinely backed up periodically. Conventionally, data is backed up to tapes by storing a copy of the data on tape. A server that runs a backup application to perform data backup (hereinafter, a “backup server”) receives data from a number of clients periodically and stores a copy of the data on tape.
However, tape drives are streaming devices that cannot be simultaneously shared by two servers without additional software, which makes the storage system more complex and expensive. For example, software may be implemented in some conventional system to multiplex or interleave multiple incoming data streams from multiple clients.
Another type of data storage devices, disks (e.g., optical disks, magnetic disks, etc.), typically allows simultaneous access by multiple servers. Moreover, disks generally are more reliable than tapes, because media failure in disks may be reduced by adopting a redundancy scheme, such as Redundant Array of Independent Disks (RAID). However, many organizations today have already invested significant resources into tape-based data backup systems. It would be costly to completely replace these tape-based data backup systems with disk-based systems. As a result, disk-based data backup systems compatible with conventional tapes are developed to accommodate organizations that prefer to retain their existing tapes. A disk-based data backup system that appears like a tape library to a backup application is referred to as a virtual tape library (VTL). An example of a VTL is the NetApp® Nearstore® VTL provided by Network Appliance, Inc. in Sunnyvale, Calif.
Currently, a VTL may be communicably coupled to a number of disks, which may be arranged in volumes. To backup data, the VTL assigns backup data to the disks. Furthermore, to better utilize the storage capacity of the disks, the VTL may compress the data. To compress the data, the VTL may store portions of the data and some information about the data, which is typically called “metadata,” instead of storing the entirety of the data, in order to reduce the storage used. For instance, one conventional way to compress data is by eliminating redundant data. If the VTL has already stored a prior version of the data in a disk, the VTL may store the differences between a current version of the data and a prior version of the data instead of storing a copy of the entire current version of the data.
However, versioning may not take into account access latency of the storage devices on which the data is stored. In general, access latency refers to the time it takes to access data in a storage device. Due to the poor access latency of disks, backing up data by versioning alone may lead to poor performance in restoring the backup data.